A transmission of this type is described in British Patent Specification GB No. 1,434,928. The purpose of the movable mounting means for the central gear is to attempt to equalize the sharing of torque transmission between the layshafts and between the layshaft gears. Thus if one of the layshaft gears is transmitting a greater torque than the other, pivoting of the central gear occurs which tends to urge the teeth of the central gear and said other layshaft gear more firmly into engagement with each other and thus increases the torque transmitted through maid other layshaft gear and reduces the torque transmitted through said one layshaft gear. In effect, a closed helicoidal torque loop is established and maintained during the transmission of power, and the transmission can be designed on the basis that the total torque transmitted will be shared approximately equally by the layshafts and layshaft gears. Without such a pivotal mounting means, inequalities of torque sharing caused by machining and/or assembly errors lead to higher maximum torques being transmitted through one or both of the layshafts, and thus a stronger construction is required.
A problem which arises due to the pivotal mounting means is that, when the central gear is pivoted from a normal position, that is to say with the plane of the gear not at right angles to the axial direction, frictional forces arising between the gear teeth act in a direction which tends to return the central gear to the normal position, especially when substantial pivoting occurs, thus preventing complete compensation for torque inequalities between the two layshafts. Furthermore, when the central gear is pivoted from its normal position, the primary pressure bearing regions of the layshaft gear teeth are laterally displaced from the centers of the teeth and perfect meshing between the teeth of the central and layshaft gears is not achieved. The gear teeth must therefore be made more robust than would otherwise be required.